In recent years there has been an increased concern in the United States and elsewhere about air pollution from industrial emissions of noxious oxides of nitrogen, sulfur and carbon. Government agencies, in response to such concerns, have in some cases already placed limits on allowable emissions of one or more of the pollutants, and the trend is clearly in the direction of increasingly stringent restrictions. Petroleum fuel refineries are particularly affected by present and anticipated restrictions on emissions, particularly emissions of nitrogen oxides and carbon monoxide. Catalytic cracking, a major petroleum refinery process, is usually the largest single source of nitrogen oxides in refineries.
The catalytic cracking of petroleum hydrocarbons to lower molecular weight products by the fluid catalytic cracking (FCC) or moving bed (TCC) processes is carried out on a large scale in petroleum refineries. In the FCC process, the cracking is carried out in a cyclic mode in which a heavy hydrocarbon feedstock such as a gas oil is contacted with hot, active, solid particulate catalyst in the absence of added hydrogen at rather low pressures of up to about 50 psig and temperatures sufficient to support the desired endothermic cracking reactions. The FCC catalyst is a fine powder of about 10 to 200 microns, preferably about 70 micron size. As the hydrocarbon feed is cracked to more valuable and desirable products, "coke" is deposited on the catalyst particles and in the course of this process, a large portion of the organic nitrogen in the feed becomes incorporated into the coke deposit. The coked catalyst particles are disengaged from the hydrocarbon products and regenerated by contact with an oxygen-containing gas in a regenerator so that the coke is burned away from the particles to restore their catalytic activity and selectivity. The heated, regenerated catalyst particles are then returned to the cracking zone and contacted with additional hydrocarbon feed and the cycle is repeated. The phrase "circulating inventory of cracking catalyst" as used here includes the total catalyst contained in the cracking unit, including the cracking and regenerator sections of the cracking plant, as well as the associated equipment including transfer lines, standpipes and the like.
Although most petroleum cracking is conducted by the fluid (FCC) process, non-fluid catalyst beds also may be used. Processes operated continuously and in cyclic fashion, with movement of the catalyst against gravity being effected, as needed, pneumatically or mechanically. One typical example of industrially practiced moving bed hydrocarbon catalytic cracking is known as Thermofor Catalytic Cracking (TCC). In this process the catalyst is in the shape of beads or pellets having an average particle size of about 1/64 to 1/4 inch, preferably about 1/8 inch. Although the present invention is described here for simplicity in terms of fluid catalytic cracking, the invention encompasses both fluid and moving-bed type processes.
In general, when the catalyst is regenerated by burning off the coke deposits with a deficiency of oxygen, the regenerator flue gas has a high CO/CO.sub.2 ratio and a low level of nitrogen oxides, but when burned with excess oxygen, the flue gas has a high level of such nitrogen oxides and a reduced CO content. Thus, catalytic cracking regenerators emit CO, or nitrogen oxides, or mixtures of these pollutants with the flue gas in varying quantities, depending mainly on feed quality and mode of operation of the regenerator as well as on the design of the unit.
When incomplete combustion of coke in the regenerator of a fluid catalytic cracking plant leaves a significant amount of carbon monoxide (CO) in the flue gas, not only is the CO released to the atmosphere, but it also tends to sporadically burn (by reaction of CO with residual oxygen) in the regenerator vessel and in the ducts and flues of the plant (afterburning), often damaging these structures by the excessive temperatures arising from the highly exothermic combustion of the carbon monoxide. Trace amounts of a platinum group metal, such as 1.0 parts per million of platinum incorporated with the cracking catalyst, are capable of effectively catalyzing the complete burning of carbon monoxide to carbon dioxide in the regenerator without detriment to the cracking reaction. This development simultaneously eliminated the environmental problem and the problem of sporadic afterburning, and has been very widely accepted by refiners. Such catalysts and their use are described in the Schwartz U.S. Pat. Nos. 4,251,395, 4,265,787, 4,088,568, 4,072,600, 4,093,535 and 4,159,239, to which reference is made for a description of these catalysts and their use in cracking operations. As described in those patents, the promoted catalysts may be used to burn the CO completely (referred to in this specification as "full CO-combustion") or only partially (referred to in this specification as "partial CO-combustion") by the simple expedient of limiting the oxygen supplied to the regenerator. The term "Pt group metal CO-oxidation promoter" as used in this specification means those metals and their mode of use, as taught, for example, by the Schwartz patents cited above. Operation with CO-combustion promoters, which solves the CO emissions problem, can in some instances increase the NO.sub.x emissions.
Although several nitrogen oxides are known which are relatively stable at ambient conditions, the nitric oxide and nitrogen dioxide which may be formed in the regenerator under appropriate conditions, are interconvertible according to the equation: EQU 2NO+O.sub.2 =2NO.sub.2.
In the present specification, NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide (the principal noxious oxides of nitrogen), as well as mixtures containing these gases.
It is generally recognized that two of these, viz. nitric oxide (NO) and nitrogen dioxide NO.sub.2), are the principal contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. Because of this, various proposals have been made for reducing the emission of nitrogen oxides from catalytic cracking units.
U.S. Pat. No. 3,900,554 (Lyon) describes a homogeneous gas phase thermal reaction to remove NO.sub.x from combustion effluent by adding 0.4 to 10 moles (preferably 0.5 to 1.5 moles) of ammonia followed by heating to 1600/C. to 2000/C. The NO.sub.x content is lowered as a result of its being reduced to nitrogen by reaction with ammonia. The so-called "selective catalytic reduction" type process which operates at a much lower temperature, 200/ to 600/C., is exemplified by U.S. Pat. No. 4,220,632 (Pence), which describes a process for reducing NO.sub.x from a fossil fuel fired power generation plant, or from other industrial plant off- gas stream, to elemental nitrogen and/or innocuous nitrogen oxides by employing ammonia as reductant and, as catalyst, the hydrogen or sodium form of a zeolite having pore openings of about 3 to 10 .ANG..
In application Ser. No. 07,433,407, we have described a method for reducing NO.sub.x emissions formed in cracking catalyst regeneration without resorting to the very high temperatures required for the gas phase thermal reaction with NH.sub.3, without the addition of ammonia, and without the necessity of providing expensive downstream facilities to clean up the flue gas. According to application Ser. No. 07,433,407, the reduction in NO.sub.X in the regeneration flue gases is effected by incorporating into the circulating catalyst inventory of the catalytic cracking unit an amount of additive particles comprising a highly siliceous crystalline zeolite containing copper metal or ions preferably in an amount equivalent to at least one half mole of CuO for each mole of Al.sub.2 O.sub.3 in said zeolite. The amount of additive particles constite 0.1 to 30 wt%, and preferably 0.1 to 10 wt% of the circulating inventory including additive and the zeolite preferably has the crystal structure of ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-38, ZSM-48, mordenite, dealuminated Y or Zeolite Beta. The preferred zeolites have a silica:alumina ratio of 20 to 100.
The advantage of the method of the invention described in Ser. No. 07,433,407 is its simplicity: no modification of the catalytic cracking equipment is required, nor is there any necessity for expensive downstream facilities for the downstream treatment of the flue gas, either to reduce NOX or carbon monoxide emissions. A further advantage of the process is that the octane of the product gasoline may be improved by the use of the preferred additive catalysts.